Transparent oxide electrode films have both high conductivity, and high transmittance of the visible light spectrum. Consequently, transparent oxide electrode films are used as the electrodes in solar cells, liquid crystal display elements, and various other photodetectors. In particular, transparent oxide electrode films formed using sputtering targets or ion plating tablets which can form transparent oxide electrode films with low resistance and high transmittance, can make sufficient use of solar energy, and are suitable for use in solar cells.
Widely used varieties of transparent oxide electrode films include tin oxide (SnO2) films doped with antimony or fluorine, zinc oxide (ZnO) films doped with aluminum or gallium, and indium oxide (In2O3) films doped with tin. Indium oxide films doped with tin, that is In2O3:Sn films, are referred to as ITO (Indium Tin Oxide) films, and are particularly widely used because of the ease of obtaining a low resistance film.
Because these films are transparent oxide electrode films which have high carrier electron concentration, and excellent reflection and absorption characteristics for near infrared wavelengths, they are also used as heat reflecting film for car window glass or building window glass, as a variety of antistatic coatings, and as transparent heating elements for defogging freezing display cases and the like.
Frequently used methods for manufacturing the transparent oxide electrode films mentioned above include sputtering methods and evaporation methods, ion plating methods, and methods involving the application of a transparent conductive layer formation coating. In particular, sputtering methods and ion plating methods are effective when forming a film on a film deposition material (referred to simply as the “substrate” below) using a material with low vapor pressure, or when precise film thickness control is required, and are widely used because of their extreme ease of operation.
In a sputtering method, generally a film is formed by causing a glow discharge to occur between the substrate acting as the anode and a target acting as the cathode, under an argon gas pressure below approximately 10 Pa, to generate argon plasma, and then causing the positive argon ions within the plasma to strike the cathode target, thereby ejecting particles of the target component, and causing these particles to be deposited on the substrate.
Sputtering methods are classified according to the method used to generate argon plasma, and those methods which use RF plasma are called RF sputtering methods, while those using direct current plasma are called DC sputtering methods. Furthermore, methods in which a film is deposited by providing a magnet behind the target and concentrating argon plasma directly onto the target, to raise the collision efficiency of the argon ions even under low pressure, are called magnetron sputtering methods. Normally, a DC magnetron sputtering method is used to manufacture the transparent oxide electrode films described above. Furthermore, sometimes plasma obtained by superposing radio frequency on a base direct current plasma is used. This is called RF superimposed DC sputtering, and enables the discharge voltage to be lowered RF superimposed DC sputtering is often used when creating an oxide film using an oxide target. Because sputtering can be performed at a low discharge voltage, the bombardment of the film by oxygen ions produced from the target can be minimized, and a good quality film can be obtained.
Here, solar cells comprise p-type and n-type semiconductors in a layered structure, and are broadly classified according to the type of semiconductor. The most commonly used type of solar cell uses silicon, which is both safe, and abundant in terms of natural resources. There are three types of solar cells using silicon, those using single cell silicon, polysilicon and amorphous silicon. Furthermore, development is proceeding in the field of solar cells known as compound thin film solar cells, which use thin films of compound semiconductors such as CuInSe2, GaAs and CdTe and the like. Some examples are as disclosed in JP Patent Publication No. Tokukai Hei 5-218479, JP Patent Publication No. Tokukai Hei 9-55526 and JP Patent Publication No. Tokukai Hei 11-145493, and the like. Regardless of the type of solar cell, it is essential that a transparent oxide electrode film be provided as an electrode on the side of the solar cell from which sunlight enters, and conventionally, an ITO film or a zinc oxide (ZnO) film doped with aluminum or gallium or the like has been used.
The following is a detailed description of the compound thin film solar cells mentioned above. The structure of a solar cell which uses a compound thin film is that of a heterojunction of a compound semiconductor thin film with a wide band gap (an n-type semiconductor middle layer) and a compound thin film with a narrow band gap (a p-type semiconductor light absorbing layer). Normally, because of the scarcity of p-type semiconductor thin films with a wide enough band gap (>2.4 eV) for use as the middle layer of a solar cell, and because of the longer minority carrier diffusion length for electrons, an n-type semiconductor is used as the middle layer and a p-type semiconductor is used as the absorbing layer. The p-type semiconductors which can be used as the light absorbing layer include CuInSe2, CuInS2, CuGaSe2, CuGaS2 and solid solutions of these compounds, and CdTe. The conditions required to obtain higher energy conversion efficiency are an optimal optical design allowing a greater number of photoelectric currents to be obtained, and the creation of high quality hoterojunctions and thin films where carrier recombination does not occur at the interface and specifically at the absorbing layer. Whether or not a high quality heterointerface is obtained depends largely on the combination of middle layer and absorbing layer, and useful heterojunctions are obtained with CdS/CdTe based, CdS/CuInSe2 based and CdS/Cu(In, Ga)Se2 based layers and the like. Furthermore, the use of a semiconductor with a wider band gap such as CdZnS as the semiconductor thin film of the middle layer in an attempt to increase the efficiency of a solar cell has yielded an improvement in sensitivity to the short wavelength light in sunlight. In addition, a high performance solar cell has been proposed which has high reproducibility due to the placement on the light incident side of a CdS or (Cd, Zn)S thin film, of a semiconductor with a larger band gap than that thin film, for example a ZnO or (Zn, Mg)O thin film, as a widow layer. Conventionally, ITO films, and zinc oxide (ZnO) films doped with aluminum or gallium or the like are employed as the transparent oxide electrode film used as the electrode on the light incident side of the cell.
The properties required of the transparent oxide electrode film used here are low resistance and high transmittance of sunlight. The spectrum of sunlight includes light ranging from 350 nm ultraviolet rays to 2500 nm infrared rays, and in order to effectively convert these forms of light energy into electrical energy, a transparent oxide electrode film is required which can transmit as wide a wavelength range as possible.
The following is a detailed description of the photo detection element. It is necessary to detect weak light for the advancement of optical communication techniques, medical diagnostics and environmental instrumentations. The detection of weak light is also required in the field of the detection of bioluminescence, the photochemical analysis of microdose agent in blood and the like, in addition to the precise spectrum analysis and the astronomic observation. In the prior art, the detection of weak light has been mainly used for the detection in and around the visible light region, however, its demand increases for the detection in the near-infrared-light region. Any of the light emission (wave length: 1.3 μm) of active oxygen, its movement in vivo being worthy of notice, the low-loss region of optical fiber (wavelength: 1.3 μm band or 1.55 μm band) and the safe region of laser wavelength to eyes (wavelength at least 1.4 μm) is the near-infrared-light region, and a variety of photo detection elements for weak infrared light is developed vigorously.
The wavelength used in the communication using optical fiber is in the infrared light region and the infrared light source of 1.3 μm band and 1.55 μm band is mainly used, and therefore photo detection elements of high-performance for infrared light are required to simultaneously detect the infrared light having such wave lengths. Generally, the photo detection element has a structure wherein the layer of photo detection materials is interposed between a pair of electrodes. As the layer of infrared photo detection materials, there are a type using Gc or InGeAs based semi-conductor materials, i.e. photodiode (PD) or avalanche photodiode (APD), the other type using materials wherein at least one element selected from the group of Eu, Ce, Mn and Cu, and at lease one element selected from the group of Sm, Bi and Pb are added to alkaline-earth metal sulfide or selenide as disclosed in JP Patent Publication No. Tokukai Hei 5-102499. APD using the multi-layered structure amorphous silicon-germanium and amorphous silicon is disclosed in JP Patent Publication No. Tokukai 2001-127336. A transparent electrode is used as an electrode on the light incident side of the photo detection element, and an ITO film has been used for it (e.g. JP Patent Publications Nos. Tokukai Hei 5-102499, Tokukai Hei 11-214737, Tokukai 2001-127336).
Generally, when light enters a substance, part of the light is reflected, part of the remainder is absorbed into the substance, and the rest is transmitted through the substance. In2O3 type and ZnO type transparent conductive materials are known as n-type semiconductors, in which carrier electrons are present, the movement of which contributes to electrical conduction. The carrier electrons in these transparent oxide electrode films reflect and absorb infrared rays. Increasing the carrier electron concentration in the film increases the reflection and absorption of infrared rays (“Transparent Conducting Film Technology”, Japan Society for the Promotion of Science, Ohm Co. Pages 55 to 57). That is, an increase in the carrier electron concentration results in a decrease in the transmission of infrared rays. So as not to decrease the transmission of infrared rays whose wavelength is 1000-1400 mm, the carrier electron concentration should be less than 5.5×1020 cm−3, and preferably less than 4.0×1020 cm−3.
Because the carrier electron concentrations of the conventionally used ITO films and zinc oxide (ZnO) films mentioned above are higher than 1×1021 cm−3, they have low resistance, but they absorb and reflect infrared rays above 1000 nm, letting hardly any through.
Furthermore, generally, the resistivity ρ of a substance depends on the product of the carrier electron concentration n and the carrier electron mobility μ (1/ρ=enμ where e is the elementary charge). In order to improve the transmittance of infrared rays, there should be as few carrier electrons as possible, but in order to minimize the resistivity ρ, the carrier electron mobility μ must be large.
The carrier electron mobility of low resistance oxide electrode films made from conventional materials is approximately 20 to 30 cm2/Vsec in the case of ITO film, for example. The carrier electron mobility in n-type semiconductors such as indium oxide (In2O3) based semiconductors is said to be governed mainly by ionized impurity scattering and neutral impurity scattering and the like (impurities included in ion form are called ionized impurities, and impurities included in a neutral form with excess oxygen adhered around the periphery are called neutral impurities). If a large volume of impure elements is added to increase the number of carrier electrons, the carrier electrons are scattered, reducing the carrier electron mobility.
Even with materials such as ITO, it is possible to reduce the number of carrier electrons and improve the infrared ray transmittance by using deposition methods for reducing oxygen deficiency, in which an increased amount of oxygen is introduced during sputtering. However, the use of this method causes an increase in neutral impurities, and a reduction in carrier electron mobility occurs as a result, causing the electrical resistivity to rise.
The use of indium oxide films doped with titanium as transparent oxide electrode films is long known in the art. For example, the oldest such instance is in the document (RCA Review, 1971 Volume 32, pages. 289 to 296) written by J. L. Vossen. This document centers on the properties of ITO films formed by RF sputtering, but also mentions an example of the manufacture of an In2O3 with 20 mol % of TiO2 added as impurities other than tin. However, the composition of this film differs markedly from the film composition of the present invention, and the electrical resistivity (resistivity) of the film is remarkably high at 7.5×10−1 Ωcm.
Furthermore, a method of manufacturing an indium oxide film containing titanium on a polyethylene terephthalate film by means of a sputtering method using an indium oxide target containing 5 mass % titanium oxide is disclosed in JP Patent Publication No. Tokukai Sho 59-204625. However, because organic polymer materials like polyethylene terephthalate melt when heated to over 70° C., specifically over the glass transition temperature, deposition by sputtering can only be performed at substrate temperatures less than 70° C., and the structure of films made under such conditions is either amorphous or amorphous with a partial crystalline phase. Although an assumption is made that if there are amorphous sections within the film then the carrier electron mobility for the film is low, this publication has no disclosure relating to carrier electron concentration, carrier electron mobility, and infrared ray transmittance.
An indium oxide target containing titanium and the characteristics of a film created from this target using a sputtering method are disclosed in JP Patent Publication No. Tokukai Hei 9-209134. The object in this publication is to provide an oxide electrode film with high resistivity for use in touch panels, and in the examples are described oxide indium films containing titanium which have high resistivity, between 1.0×10−3 and 9.4×10−3 Ωcm. Furthermore, the indium oxide film containing titanium which has the lowest resistivity of those in the comparative examples has resistivity of 0.6×10−3 Ωcm. The resistivity for either case is quite high.
There are several patent applications relating to film materials based on ITO to which titanium is added. However, these are clearly distinguishable from the film of the present invention in that they contain tin. In other words, it is conventionally known that when tin is included in an indium oxide film, a large number of carrier electrons are released, and as such the carrier electrode concentration is high, allowing only films with low transmittance of the infrared light region to be obtained. For example, an indium oxide film containing titanium and tin for use in touch panels is disclosed in JP Patent Publication No. Tokukai Hei 9-161542. However, the resistivity as described in this publication is high at 9.6×10−4 Ωcm.
Furthermore, a film of ITO or of indium oxide to which titanium is added is also disclosed in JP Patent Publication No. Tokukai Hei. 6-349338. However, because polyethylene terephthalate, a molded organic polymer, is used as the substrate, the use of sputter deposition methods which involve heating is problematic. If sputter deposition is performed at temperatures low enough to not cause organic polymers to melt (less than 70° C.), normally only a completely amorphous film structure or an amorphous structure including a partial crystal phase can be obtained, which is clearly different from a crystalline film. Furthermore, the sheet resistance and the film thickness are disclosed, but the resistivity calculated from these values (resistivity=sheet resistance×film thickness) is a high value.
An ITO sintered body target (10 mass % SnO2) to which 50 to 500 ppm of Ti is added in order to raise the density of the sintered body is described in JP Patent Publication No. Tokukai Hei. 7-54132, and also described therein is an oxide electrode film with low resistance of between 1.7×10−4 Ωcm and 2.9×10−4 Ωcm manufactured by performing sputtering using this target while heating the substrate to between 100 and 300° C. However, there is no disclosure referring to the carrier electron concentration, carrier electron mobility or infrared light transmittance of this film, but an assumption can be made that because an ITO based material is used, most of the added elements which contribute to carrier electron production are tin, and the carrier electron concentration is high in line with conventional ITO, and consequently the infrared light transmittance is low.
On the other hand, the inventors of the present invention have proposed an indium oxide material containing tungsten as an indium oxide based transparent oxide electrode film with high transmittance of infrared light, in Japanese Patent Application No. 2002-200534. In this application, a case is disclosed in which an transparent oxide electrode with high infrared light transmittance and low resistance is manufactured with the substrate temperature between 200° C. and 300° C. It is shown that when the transparent oxide electrode film as disclosed in this application is deposited at low temperatures, for example 100° C. or 150° C., the resistivity is increased. In the manufacture of transparent oxide electrode film by means of a sputtering method, because a shortening of the heating time and a reduction in the electric power used to heat the substrate can be realized by using lower substrate temperatures, there remains problems with this invention in terms of manufacturing costs and productivity.
However, a low resistance indium oxide containing both titanium and tungsten has not been reported to date.
Although the ITO films and zinc oxide (ZnO) films described above have low resistance and high transmittance in the visible light region, their transmittance in the infrared light region is low, and the solar cells using these films as an electrode on the light incident side is not sufficient to use the solar energy in the infrared light region. Moreover, the same phenomenon can be said on the photo detection elements, and the rate of the infrared light entering in the layer of photo detection materials is low and the sensibility consequently becomes wrong in the conventional photo detection elements in which the ITO films and zinc oxide (ZnO) films described above are used as an electrode on the light incident side. Because the reflection and absorption of infrared light by a transparent oxide electrode film increases the higher the carrier electron concentration becomes, it is thought that the reason for the low transmittance of the infrared spectrum of these ITO films and zinc oxide (ZnO) films is that as the converse effect of the low resistance, the carrier electron concentration is high.